Vendor enum34
Vendor enum34

This is needed for https://github.com/dpkp/kafka-python/pull/1540

While the usage there is trivial and could probably be worked around, I'd
rather vendor it so that future code can use enums... since `enum` is
already available in the python 3 stdlib, this will be easy enough to
eventually stop vendoring whenever we finally drop python 2 support.

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